There are many processes which involve contact between an elastic fluid, such as a gas or vapour, and a particulate solid. Thus many chemical processes are carried out using gas phase or vapour phase reaction conditions in which a gas or vapour stream is contacted with a particulate catalyst. Other processes in which an elastic fluid is contacted with a particulate solid include drying, in which a gas or vapour is contacted with a desiccant, and adsorption, in which a gas or vapour is contacted with an absorbent for the purpose of, for example, adsorption of potential catalyst poisons therefrom.
In such processes the particulate catalyst or other particulate solid is frequently in the form of a fixed bed, although some processes are operated using a fluidised catalyst bed.
The conditions used in such processes often include high operating temperatures and/or high pressures. Hence reactors may have to withstand high thermal and pressure stresses. Typical constructional materials for chemical process vessels accordingly include mild steel, high pressure steel, stainless steel and other special steels and alloys.
The use of catalysts, supported catalysts and other particulates, such as desiccants and adsorbents, in fixed bed applications is thus widespread. The particulate matter forming the fixed bed is typically ceramic in nature or formed from pelletised metal oxides. Usually it has a lower coefficient of expansion than the reactor, tube or other containment device for the particulate solid which is often composed of metal for pressure strength reasons. Thus, when the system increases in temperature, the particulate material slumps in the reactor because, upon heating, the walls of the reactor expand more than do the catalyst particles. Then when the temperature is later lowered, the walls of the reactor contract as it cools and the particulate matter may be caught as if by a tightening corset and thereby subjected to a crushing force, particularly if the particulate solid is contained in a substantially vertical metal tube.
In many applications the temperature variations in operation are not very high and the different amounts of expansion between the particulate matter and the containment device are not significant. Consequently excessive attrition of the particulate material or damage to container walls is not caused. However, in so-called fired processes which utilize high temperature operations, typically involving combustion in order to maintain the temperature in endothermic catalytic processes such as steam reforming, or in exothermic catalytic processes such as partial oxidation processes, the amounts of expansion involved are considerable. If the fixed bed is contained in a large diameter reactor or containment device, this differential expansion can be accommodated with only minor attrition of the catalyst particles since there are many particles and cumulative small movements of the catalyst particles into internal voidage will occur. However, if the catalyst particles are contained in a narrow vertical tube having, for example, a nominal diameter of less than about 6 inches (about 15.24 cm), this relative movement is insufficient and very high crushing forces can be generated. This tends to result in attrition of the particulate matter, if friable to any degree, or in damage to the tube wall, if not. The latter phenomenon has been observed with physically strong alumina catalyst support balls in high temperature reformer tubes. Furthermore, in cases where the vertical tubes are very long and experience considerable expansion over their length due to the high operating temperature being used, for example steam reformer tubes, the particulate matter drops by a very significant amount but cannot rise back up the tube when it cools due to being tightly squeezed by the cooling tube, a factor that exacerbates the crushing tendency.
Repeated heating and cooling cycles lead to a deterioration in the desired characteristics of the packed bed, in that the originally loaded volume of particulates is compressed to a higher density, thereby increasing the pressure drop. In addition it has been found that increased pressure drop through a catalyst bed can be caused by, amongst other reasons, breakage of catalyst particles resulting from incorrect charging of the catalyst or from differential expansion and contraction between the catalyst and the containing vessel due to temperature cycling at start-up and shut-down. The breakage of catalyst particles gives fragments of a smaller particle diameter, while erosion of the corners of particles gives a lower voidage due to the eroded particles packing more closely together. For further discussion reference may be made to “Catalyst Handbook”, 2nd Edition, by Martyn V. Twigg (Wolfe Publishing Ltd., 1989), at page 125. This increased pressure drop generally increases the costs associated with gas compression in all fixed bed applications. In parallel fixed bed applications this can lead to increasing maldistribution, especially in a multi-tubular reactor, thereby causing different conversions and selectivities in different tubes. This, in turn, can lead to further problems such as carbon laydown, formation of hot spots (leading to possible tube failure and/or to sintering of the catalyst), and to development of different rates of catalyst deactivation which can further exacerbate the situation. Loss of catalyst surface material by spalling and attrition is particularly serious when the active part of the catalyst is in the form of a shallow surface layer, because in this case considerable catalyst activity can be lost or the catalyst activity can become maldistributed.
The debris from the crushing forces will accumulate in the, by now, more dense bed and also increase the pressure drop. There will be an increased likelihood of different pressure drops between different tubes in a multi-tubular reactor leading to maldistribution of the gas or vapour. In addition, the position of the top of the bed within any individual tube will be difficult to predict.
Another problem occurs with externally fired tubular reactors, such as reformers, in that any part of the tube that does not contain catalyst is liable to overheat, with a consequent danger of tube failure, since there is no endothermic reaction being catalysed in that part of the tube to absorb the radiant heat and hence to cool that part of the tube. This makes it important to determine as closely as possible the position of the catalyst bed during operation so as to minimize the risk of tube failure through local overheating.
There is, therefore, a need in the art to provide a reactor design which overcomes the problems associated with crushing of particulate materials when the reactor is subjected to temperature cycles of heating to high temperatures followed by cooling again, and which allows low pressure drop through the particulate material, minimizes pressure drop build-up, and allows the position of the bed to be fixed with a high degree of certainty so as to minimize the risk of tube failure in an externally fired reactor.
This need has been recognized previously and there are various examples in the prior art of attempts to overcome the problems outlined above.
The crushing of catalysts by radial forces due to wide temperature cycles in tubular reactors, such as steam reforming reactors, has been recognized in U.S. Pat. No. 4,203,950 (Sederquist). In this document it is proposed that the catalyst should be arranged in an annulus with at least one wall being flexible.
In U.S. Pat. No. 5,718,881 (Sederquist et al.) a steam reformer has segmented reaction zones with individual supports for different temperature zones, the volume of the segments of catalyst being inversely proportional to the temperature of the various zones in the reformer.
The use of flexible louvered screens to accommodate particle movement is proposed in U.S. Pat. No. 3,818,667 (Wagner). Louvers are also proposed in a catalytic converter for catalytically treating the exhaust gases from an internal combustion engine in U.S. Pat. No. 4,063,900 (Mita et al.), and in U.S. Pat. No. 4,052,166 (Mita et al.).
It is proposed in U.S. Pat. No. 3,838,977 (Warren) to use springs or bellows in a catalytic muffler to control bed expansion and contraction so as to maintain a compacted non-fluidised or lifted bed. Spring loading to maintain a bed of carbon granules tightly packed within a fuel vapour storage canister housing is described in U.S. Pat. No. 5,098,453 (Turner et al.).
A ratchet device to follow the decrease in volume of a bed but restrain back-movement of an upper perforated retaining plate is proposed in U.S. Pat. No. 3,628,314 (McCarthy et al.). Similar devices are described in U.S. Pat. No. 4,489,549 (Kasabian), in U.S. Pat. No. 4,505,105 (Ness), and in U.S. Pat. No. 4,554,784 (Weigand et al.).
Pneumatic sleeves inside a catalyst bed to restrain movement of the particulate material are proposed in U.S. Pat. No. 5,118,331 (Garrett et al.), in U.S. Pat. No. 4,997,465 (Stanford), in U.S. Pat. No. 4,029,486 (Frantz), and in U.S. Pat. No. 4,336,042 (Frantz et al.).
However, these prior art proposals are elaborate and do not solve satisfactorily the problem of crushing of particulate catalysts which can be caused by repeated temperature cycling of a reactor tube.
Catalysts are usually passed over a screen to remove dust and broken pieces either before shipment and/or before loading into a reactor. Such removal of dust and broken pieces of catalyst is desirable in order to minimize the pressure drop across the reactor caused by the catalyst bed. This screening step constitutes a costly procedure both in terms of finance and time. Once loaded, catalyst particles usually cannot be re-arranged and the packed density only tends to increase.
The loading of catalysts can be achieved by a number of methods to reduce breakage and damage caused by free fall loading. For example, “sock” loading can be used in which the catalyst is put into long “socks”, usually made of fabric, which are folded or closed at one end with a releasable closure or tie which can be pulled to release catalyst when the sock is in position. Another method, which is more suitable for use in forming beds in vessels of large diameter, for example from about 0.75 m to about 4 m or more in diameter, than for loading tubes of diameter less than about 25 cm, is so-called “dense” loading in which the catalyst is fed through a spinning distributor so as to lay down consecutive level layers rather than mounds of dumped catalyst. A third method, which is suitable for loading vertical tubes, utilizes wire devices or wires in tubes which reduce falling velocities. One option is to utilize one or more spirals of wire inside the tube so that the catalyst particles bounce their way down the tube and do not undergo free fall over the full height of the tube. As the tube is filled, so the wire or wires is or are withdrawn upwardly, optionally with vertical fluctuations. Such devices are proposed, for example, in U.S. Pat. No. 4,077,530 (Fukusen et al.).
A further possibility is to use a line having spaced along its length a series of brush-like members or other damper members and to withdraw the line upwardly as the catalyst particles are fed into the tube, as described in U.S. Pat. No. 5,247,970 (Ryntveit et al.).
“Sock” loading can also be carried out semi-continuously in large diameter vessels with a funnel and a filled fabric or solid tube which is moved and raised to release the catalyst with frequent leveling of the catalyst.
Each method of loading produces fixed beds with different bulk densities. The density differences can be quite marked; for example, with cylindrical particulate materials or extrudates the “dense” loaded density can be as much as about 18% greater than the corresponding “sock” loaded density due to the particles being laid generally horizontally and parallel to each other in the “dense” method rather than at random following “sock” removal.
In some applications it is desirable to maximize the amount of catalyst loaded, despite increased pressure drop through the fixed bed, in which case “dense” loading or loading into liquid may be used and/or the tubes may be vibrated.
U.S. Pat. No. 5,892,108 (Shiotani et al.) proposes a method for packing a catalyst for use in gas phase catalytic oxidation of propylene, iso-butylene, t-butyl alcohol or methyl t-butyl ether with molecular oxygen to synthesise an unsaturated aldehyde and an unsaturated carboxylic acid in which metal Raschig rings are used as auxiliary packing material.
In U.S. Pat. No. 5,877,331 (Mummey et al.) there is described the use of a purge gas to remove fines from a catalytic reactor for the production of maleic anhydride which contains catalyst bodies. In this procedure the purging gas, such as air, is passed through the catalyst bed at a linear flow velocity sufficient to fluidise the catalyst fines but insufficient to fluidise the catalyst bodies. At column 15 lines 16 to 18 it is said:                “In order to prevent fluidization or expansion of the catalyst bed during further operation of the reactors, and in particular to prevent the catalyst bodies in the fixed catalyst bed from abrading against one another or against the tube walls, a restraining bed comprising discrete bodies of a material substantially denser than the catalyst was placed on top of the column of catalyst in each tube of the reactors.”It is also taught that this upflow removes undesirable fine particles which, if left in the densely packed vessel, may contribute to plugging of the bed.        
In U.S. Pat. No. 4,051,019 (Johnson) there is taught a method for loading finely divided particulate matter into a vessel for the purpose of increasing the packing density by introducing a fluid medium counter-current to the downward flow of the finely divided particulate matter at a velocity selected to maximize the apparent bulk density of the particulate matter in the vessel. It is taught that this method also provides a method of removing undesirable fine particles which, if left in the densely packed vessel, might contribute to plugging of the bed.
Vibrating tubes with air or electrically driven vibrators and/or striking with leather-faced hammers is described in the afore-mentioned reference book by Twigg at page 569, the latter being used to further compact the catalyst in those tubes showing low pressure drop in multi-tube applications, in order to achieve equal pressure drops in each tube.
An upflow tubular steam reformer is described in U.S. Pat. No. 3,990,858 (O'Sullivan et al.). In this proposal fluidisation of the particulate material in the catalyst tubes is prevented by providing a weighted conically shaped hollow member which rests on top of the bed of particulate material. This conically shaped hollow member is provided with elongated slots whereby fluid exiting from the bed flows into the interior of the hollow member, through the slots and into the tube outlet.
There is a need to obviate in a simple and reliable way the problems caused by crushing or attrition of particulate materials, such as catalysts, desiccants or adsorbents, which are subjected to cycling between high and low temperatures in vessels, particularly vessels made of relatively high thermal expansion materials, such as steel or other metals or alloys. There is also a need to provide a method of operating a catalytic reactor in which the pressure drop across a catalyst bed can be reliably minimized in operation. In addition there exists a need for a method of loading a tubular reactor with a particulate material, e.g. a particulate catalyst, in which the presence of “fines” can be substantially avoided in the catalyst tube. Furthermore there exists a need for a method of operating a reactor containing a charge of a particulate material in which any “fines” which may be formed during the course of extended operation of the reactor can be removed simply from the reactor without having to discharge the charge of particulate solid from the reactor. There is also a need for operating a tubular reactor in which the position of the top of the bed of catalyst or other particulate material in the or each tube is known with certainty.